This invention relates to dispensing systems for mixing and dispensing multicomponent curable compositions, such as polymerizable epoxy resins formed by a mixture in prescribed ratio of two or more materials such as a base material and an accelerator material. More specifically, this invention relates to apparatus and method for preventing waste of any portion of the composition or material components thereof during start-up, normal operation, or shut down procedures.
Dispensing systems for mixing and dispensing multi-component curable compositions are well known in the art. Such systems typically include, for example, appropriate pumping mechanisms for pumping and metering separate materials, such as a base material and an accelerator material, in a prescribed ratio to a mixing device where these materials are thoroughly mixed together. When mixed, the two materials interact with each other to create a flowable, curable composition which will set up or harden to a nonflowable state in a relatively short period of time, referred to frequently as the "cure" time. Examples of such compositions are polymerizable resins and the like wherein the base material comprises a substance including unreacted polymers such as a polysulfide resin together with diluting solvents such as toluene for viscosity control, and wherein the accelerator material comprises a polymerization initiator such as an appropriate oxidizer which provides the desired chemical reaction when mixed with the base material. Various other chemical components and combinations of components may be included in either the base material or the accelerator material to adjust resultant physical properties of the mixed composition, and environmental parameters such as temperature can be controlled to increase or decrease cure time, as desired. In any event, the mixed composition must be supplied promptly from the mixing device to an appropriate dispensing nozzle for application to a surface before the composition cures.
Curable compositions of the general type described are used, for example, as adhesives or sealants in a wide variety of industrial applications. The mixed composition is applied from the dispensing system through the dispensing nozzle directly to the surface or point of application where the composition is desired. In a common assembly line type operation, the flow of the mixed composition is necessarily required to be intermittent as the composition is applied to production items in sequence passing along the assembly line, in order to prevent substantial waste of the composition as the dispensing nozzle is moved from one production item to another.
In production-type situations, it is highly desirable for the composition to cure as radidly as possible so that subsequent production operations can be performed without significant time delay. Exemplary methods of substantially decreasing the cure time of the composition are appropriate adjustment of the mixing ratio of the two materials or elevation of the temperature level of the mixed composition. However, when a desirably rapid curing composition is used, the intermittent dispensing nature of the production environment substantially increases the likelihood of the composition reaching a nonflowable cured state before exiting the mixing device or the dispensing nozzle, thereby clogging the system. This clogging problem is significantly compounded by normal production line interruptions resulting from personnel rest periods, parts or product shortages, machine breakages, and the like. Unfortunately, once the system is clogged, it can be unclogged only be time-consuming disassembly and cleaning of the clogged system components.
In the prior art, it is therefore common practice to utilize curable compositions having relatively long cure times, such as twelve to twenty-four hours, to avoid clogging of the composition dispensing system. This practice, however, requires the production items to undergo relatively expensive heating processes to cure the composition if rapid cure times are desired. Alternately, the production items must be stored in a suitable holding zone until complete curing at ambient temperature is achieved, but this latter alternative unnecessarily occupies floor space in an industrial facility which is desirably utilized for more productive purposes.
Some prior art dispensing systems have been proposed to include features for purging mixed composition from the system whenever dispensing of the composition is halted for a period of time sufficient to create clogging difficulties. For example, so-called solvent purge systems are known wherein a suitable solvent is supplied to the mixing device for flow therethrough and further through the dispensing nozzle to flush and wash the composition therefrom. The resultant contaminated solution of solvent and composition is discarded, and additional base material and accelerator material are not supplied to the mixing device until additional composition is required.
Solvent purge systems are disadvantageous in that they require relatively large quantities of relatively expensive solvents, typically petroleum-based solvents such as methylene chloride or the like. This type of solvent, as well as the flushed composition, comprise toxic waste substances which are not easily or inexpensively discarded. Moreover, when the dispensing is subsequently restarted, an initial portion of the mixed composition tend to be "off-ratio" and may include undesirable traces of solvent, thereby requiring this initial portion of the composition also to be wasted until a noncontaminated homogeneous mixture is achieved.
Another purge system proposed in the prior art comprises a so-called base purge system for flushing mixed composition from the mixing device and the dispensing nozzle. In this type of system, the accelerator material flow to the mixing device is ceased whenever purging is required, and the base material flow is continued for a period of time sufficient to flush completely the mixed composition from the mixing device and the dispensing valve. The flushed material which includes a high proportion of base material is discarded, with the unmixed and uncurable base material in the mixing device preventing clogging of the system. See, for example, U.S. Pat. No. 3,989,228.
Base purge systems are also disadvantageous in that the "off-ratio" flush material which is wasted is both expensive and toxic. Moreover, when the system is restarted, the initial mixed composition remains "off-ratio" for a substantial period of time until the unmixed base material is flushed completely from the mixing device and the dispensing nozzle. This undesirably and significantly increases the quantity of expensive waste material which must be properly disposed.
The present invention overcomes the problems and disadvantages of the prior art by providing an improved system for dispensing curable compositions. The system includes apparatus and method for preventing waste of any portion of the component materials while allowing the system to be started up and shut down as frequently as necessary.